1. Field of the Invention
The present invention relates to the field of measuring and manufacturing optical surfaces. In particular the invention relates to an interferometer apparatus for measuring an optical surface and to a method for qualifying and manufacturing an optical surface by using the interferometer apparatus.
2. Brief Description of Related Art
The substrate having the optical surface is, for example, an optical component such as an optical lens or an optical mirror used in optical systems, such as telescopes used in astronomy, or systems used for imaging structures of a mask (“reticle”) onto a radiation sensitive substrate (“resist”) in a lithographic method. The success of such an optical system is substantially determined by the precision with which the optical surface can be machined or manufactured to have a target shape. In such manufacture it is necessary to compare the shape of the machined optical surface with its target shape, and to determine differences between the machined and target surfaces. The optical surface is then further machined at those portions where differences between the machined and target surfaces exceed e.g. a predefined threshold.
Interferometric apparatuses are commonly used for high precision measurements of optical surfaces. Examples of such apparatus are disclosed in U.S. Pat. No. 4,732,483, U.S. Pat. No. 4,340,306, U.S. Pat. No. 5,473,434, U.S. Pat. No. 5,777,741, U.S. Pat. No. 5,488,477, which documents are incorporated herein by reference.
It has been found that results achieved with the conventional interferometric apparatus and method are not completely satisfactory, in particular when the optical surface to be measured has a relatively strong curvature, i.e. a lower modulus of the radius of curvature (curvature=1/radius). An example of a high precision measurement of a spherical surface according to the prior art is illustrated with reference to FIG. 1 in the following.
In this example an interferometric apparatus 1 of the Fizeau-type is used. The apparatus comprises a laser light source 3 emitting a beam of measuring light 5 which is linearly polarized by a polarizer 7 and thereafter passed through a quarter wave plate 9 such that the beam 5 downstream of the quarter wave plate 9 is circularly polarized. A focussing lens 11 provides a focus of the beam within a pinhole of an aperture 13 such that a diverging beam 15 of measuring light is formed. In interferometer optics 17 transforms diverging beam 15 to a strongly converging beam 19 such that a focus 21 of the measuring light is formed on an optical axis 23 of interferometer optics 17. Wavefronts in converging beam 19 are substantially spherical wavefronts. The interferometer optics 17 comprises a collimating lens 25 forming a parallel beam 26, and a further focussing lens 27. Lens 27 has a concave spherical surface 28 having a center of curvature which coincides with focus 21. Surface 28 is used as a reference Fizeau surface of the interferometer apparatus 1, i.e. wavefronts of the measuring light reflected from surface 28 travel back the beam path, are reflected from a semitransparent mirror 30 and imaged onto a light sensitive substrate 31 of a CCD-camera using a camera optics 32.
A lens 33 having a convex spherical surface 34 to be measured is positioned in converging beam 19 such that a center of curvature of spherical surface 34 substantially coincides with focus 21. Wavefronts reflected back from surface 34 are also imaged on detector 31 and form an interference pattern with the wavefronts reflected back from surface 28 on the detector. A first measuring result W1 obtained by camera 31 may be written asW1=WR+WT+WS,  (1)wherein WR is a reference arm wavefront, WT is a test arm wavefront and WS is the test surface wavefront (see also Daniel Malacara, Optical Shop Testing, 2nd edit-ion, Wiley interscience Publication (1992)).
Thereafter lens 33 is rotated by 180° about optical axis 23 such that a mark 35 shown in FIG. 1a at the top of lens 33 is at the bottom as shown in FIG. 1b. A second measurement obtained in this position may be written asW2=WR+WT+WS180°,  (2)wherein WS180° represents the test surface wavefront with the test surface rotated by 180°.
In a third measurement lens 33 is positioned such in converging beam 19 that the vertex of surface 34 is positioned at the focus 21. Such arrangement is also referred to as “cat's eye” arrangement. A third measuring result may be written as
                              W          3                =                              W            R                    +                                    1              2                        ⁢                                          (                                                      W                    T                                    +                                      W                    T                                          108                      ⁢                      °                                                                      )                            .                                                          (        3        )            
From W1, W2 and W3 the desired test surface wavefront may be calculated as
                                          W            S                    =                                    1              2                        ⁢                          (                                                W                  1                                +                                  W                  2                                      180                    ⁢                    °                                                  -                                  W                  3                                -                                  W                  3                                      180                    ⁢                    °                                                              )                                      ,                            (        4        )            wherein W2180° represent the images W2 and W3, respectively, which are numerically rotated by 180°.
The interferometer errors can be written as
                              W          1                =                              1            2                    ⁢                                    (                                                W                  1                                -                                  W                  2                                      180                    ⁢                    °                                                  +                                  W                  3                                +                                  W                  3                                      180                    ⁢                    °                                                              )                        .                                              (        5        )            
With the above method it is possible to determine deviations of surface 34 from its spherical target shape, and it is also possible to calibrate the interferometer optics by determining deviations of the wavefront shapes in converging beam 19 from their desired spherical shape and determining deviations of the Fizeau surface 28 from its spherical shape.
As already mentioned above, also the interferometric apparatus and method illustrated with reference to FIG. 1 has drawbacks when the measured surfaces have a large diameter and a small modulus of the radius of curvature. This results in an insufficient quality of a manufactured optical component.